Intramolecular diphosphonic acids and esters of cycloalkanes, particularly para-menthane, have been disclosed in U.S. Ser. No. 924,165, now U.S. Pat. No. 4,183,879. In general, these compounds are prepared by reacting an unsaturated cyclohexyl material, or material convertible to a cyclohexyl material, such as alpha or beta-pinene, with a dialkyl hydrogen phosphite under effective free radical conditions. Because these prior materials are useful starting materials for making the novel pyrophosphonates hereof, the disclosure of said patent is included herein by reference thereto.
In other specific aspects, starting compounds for the present invention are conveniently prepared by adding a dialkyl hydrogen phosphite to a para-menthenephosphonate. This reaction is conducted under free radical conditions at temperatures of about 50.degree. C. to about 200.degree. C. for about 0.1 to 10.0 hours until the diphosphonate is formed. The precursor monophosphonate adducts can be prepared according to procedures reported by Kenney, et al., in J. Org. Chem., Vol. 30., No. 5., pages 682-686 (1974) and by Francois, et al., in C. R. Acad. Sci., Ser. C., Vol. 279., No. 3., pages 117-119 (1974).
Alternatively, diphosphonate, or diphosphonic acid ester starting materials for the present invention can be directly prepared by reacting unsaturated cyclohexyl materials such as beta-pinene, with a dialkyl hydrogen phosphite at a temperature in the range from 50.degree. C. to 200.degree. C. for a reaction time of from 0.1 to 10.0 hours, in the presence of a free radical initiator. A wide variety of conventional free radical initiators may be used. Typical free radical initiators are various peroxides, such as, ditertiary-butyl peroxide, hydrogen peroxide, benzoyl peroxide, and the like. Generally from about 0.1 to about 10 mole percent of the free radical initiator on the basis of the hydrocarbon is used in the reaction mixture.
Additionally, ultraviolet radiation may serve to establish the free radical conditions, including the situation when a UV photoinitiator is added to the reaction mixture. Atmospheric pressure is desired for use in producing the diphosphonates. ##STR1##
The pyrophosphonates of the present invention can be produced from diphosphonate esters such as those produced in accordance with the above U.S. Pat. No. 4,183,879 by heating at a temperature of from 200.degree. C. to 300.degree. C., preferably between 240.degree. C. and 260.degree. C. for a period of from 0.5 to 10 hours.
The closest known reference which has come to my attention is European Patent No. 00043 which discloses propane-1,3-dimethylphosphinic acid anhydride having the formula: ##STR2## By contrast, in my novel compositions the radical attached to phosphorus and in the dotted area is alkoxy instead of alkyl, making the compositions phosphonates rather than phosphinates.
Another reference is the article by Gazizov, et al. in Zh. Obshch. Khim 1976, 46 (6), 1223-8, which discloses the reaction of dialkylchlorophosphites with acylals of acetic acid. This reaction gives a complex mixture of phosphorus-containing products whose structures are determined by spectroscopic and chromatographic methods. Thus, reaction of (EtO).sub.2 PCl with MeCH(OEt)OAc at 50.degree. C. for 5 hrs. gave (EtO).sub.2 P(O)CHMeOEt, (EtO).sub.2 P(O)Ac and Cl(EtO)P(O)CHMeOEt. Prolonged heating (.about.40 hr.) produces among other products [CH.sub.3 CH(OEt)(EtO)P(O)].sub.2 O, a pyrophosphonate albeit an intermolecular one. It is significant that, according to the authors, this pyrophosphonate is not produced by direct thermal reaction of phosphonates as are the products of this invention.
Reference may also be made to U.S. Pat. No. 3,597,511 which discloses P-O-P compounds produced by reacting P.sub.2 O.sub.5 with certain alkylene oxides.
French Pat. No. 1,281,973 discloses the preparation of certain aromatic pyrophosphates containing a P-O-P group. These again are not intramolecular pyrophosphonates.